1. Field of the Invention
The present invention relates to a digital phase locked loop circuit for compact-disc players and, in particular, is well applicable to prevention of a malfunction generated by wave distortion which is, in turn, caused by asymmetry.
2. Description of the Prior Art
As is commonly known, a signal read from a compact disc may contain asymmetry Such asymmetry is caused typically by the fact that the sizes and shapes of the pits engraved on the compact disc vary depending upon a variety of conditions such as the power of the light used for mastering and the phenomenon time.
Let certain part of a signal read from a compact disc comprise a component generated by a disc portion with a pit length of, for example, 3T and a non-pit area also with an equal length of 3T, and another component coming from another disc portion with a pit length of 4T and a non-pit area also with an equal length of 4T, where T is a channel-clock period. Note that there are nine different pit lengths. This means the such pit-length components of the signal each have a duty cycle of 50%.
If all parameters are perfectly controlled, a pit with a duty cycle of 50% can be accurately engraved on tracks of the compact disc. If the parameters are shifted out off the perfectly controlled states even slightly, however, a deviation is resulted in so as to gradually lenghten or shorten the pit by an equal amount in the longitudinal direction of the pit, giving rise to inconvenience.
The phenomenon described above is called asymmetry. At the present stage of technology, the asymmetry phenomenon cannot be eliminated completely during the manufacturing of the compact disc. In addition, asymmetry of the signal itself which is read by the pickup varies with the wavelength of the laser being used.
The asymmetries generated as described above give rise to discrepancies which appear as phase and frequency jitters in the phase locked loop circuit. As a result, a worsened error rate may be brought about. In particular, in a locked state of the phase locked loop circuit with a frequency deviation remaining in the EFM (Eight-to-Fourteen Modulation) signal, the effect of the asymmetries is very great, causing inconvenience.
In the case of a compact-disc system, by the way, the length of the smallest pit is of the order of 0.86.mu.m. Such a length is sufficiently big in comparison to the width which is only 0.5.mu.m. There is almost no case in which a short pit has a narrowed width in comparison to a long pit due to the effect of the asymmetry in the transversal direction. That is to say, the asymmetry has a uniform effect in the transversal direction on all pits regardless of their lengths. As for the effect in the longitudinal direction, the asymmetry lengthens or shortens the pits of all lengths by an equal amount as is described previously. In the case of a disc adopting the CD technique, the effect of the asymmetry is made fall within a certain range. In addition, a compact-disc player incorporates a circuit for automatically compensating for the effect of the asymmetry. In this way, the inconvenience caused by the asymmetry is avoided.
That is to say, the digital phase locked loop circuit incorporated in a compact-disc player carries out phase control and frequency control independently of each other. The frequency control measures the length of each pattern of the EFM signal, outputting a frequency deviation .DELTA.f. The frequency deviation is then added to the playback-clock as an offset.
On the other hand, the phase control simply detects the difference in phase between the EFM signal and the playback clock, feeding back the difference to the playback clock as a correction quantity thereof.
The frequency is computed by measuring the wavelength of each pattern and applying a predetermined weight to the difference between the measured value and a standard length. Note that the predetermined weight is applied because even for the same measurement value, the frequency differs from pattern to pattern. The weighted differences are then integrated to give a frequency deviation. It is obvious that frequency variations of even small patterns such as the 3T and 4T components are recognized as a large frequency deviation. As a result, the effect of wave distortion due to the asymmetry is great, giving rise to, among other things, an uncontrolled state out off the phase locked loop.